Rechargeable, or secondary, batteries are typically multicell series-connected to produce practical output voltages. During charging of the battery, the low capacity cells will charge to capacity first and then overcharge. If this occurs while overall battery voltage is still below the end-of-charge threshold of the battery, and if the cell chemistry has poor inherent overcharge tolerance, the battery will be damaged. During discharge, the lowest capacity cells will be exhausted while the other cells still maintain charge. Power will continue to be delivered by the charged cells, but the exhausted cells will reverse, resulting in a decrease in battery capacity.
Lead-acid and sealed NiCd batteries have tolerance to mild overcharging through inherent absorption of overcharge by competing chemical reactions. These types of batteries thus automatically balance the overcharge, becoming somewhat self-leveling (or charge-equalizing). This is an important factor contributing to the commercial popularity of these types of batteries because the battery can then be treated as a single unit which does not require monitoring of each individual cell.
During discharge, if the use of the batteries involves deep discharges, then in order to avoid discharge reversal it is necessary to match the capacities of the individual cells. However, in NiCd batteries, even after matching the cells, periodic cell-by-cell deep discharges are needed to realize full cell output and lifetime.
Silver-zinc batteries are significantly more powerful than NiCd batteries (producing approximately three times the power per pound on discharge). This type of battery has not been commercially popular primarily because of its extremely low tolerance to overcharge. That is, if overcharged, silver-zinc batteries self-destruct through disordered charges in the cells, rendering the battery useless. This extreme sensitivity to overcharge thus requires a careful monitoring of each individual cell during charging to insure that no cell is overcharged. For a typical silver-zinc battery maintenance interval of thirty charge/discharge cycles, it is prohibitively cumbersome and troublesome to charge each cell individually.
Prior art approaches to voltage monitoring of individual cells have used complicated circuits connected to each cell. Many components were necessary for each cell because of the widely different possible potentials of the cells. Further complication was added because if the battery itself is used to power a monitoring circuit, cells near the positive end of the battery require a different type of circuit from those cells near the negative end. Prior art devices also typically require a multipole disconnect circuit to avoid self-discharge of the battery through the devices when they are on standby.